Marshall:
If I were you I would not attempt to do what you propose to split the range into detection with two 8-bit converters all for the sake of having an I²C interface on the parts. Here are links to two nice 12-bit A/D converters that use the type of serial interface that is common for parts of this ilk. They use a serial interface, but it is a simple clock/data type. These interfaces are extremely easy to bit-bang with software and are much easier to electrically/optically isolate than I²C would be.

The clocking scheme also permits extremely efficient access to the conversion results in a way that would not be possible with the byte-centric nature of a pure I²C interface protocol.

* DIP8 and SOIC8 packages....
http://www.linear.com/pdf/128698fs.pdf

* 8 lead SOT23 package
http://www.analog.com/UploadedF..._50A_0.pdf

Michael Karas